1. Field of the Invention
The present invention relates to a bumper for vehicles including a reinforcement beam made of metal and a pair of support members having a hollow cross section. The pair of support members protrudes from a vehicle frame and supports the reinforcement beam therebetween.
2. Description of the Related Art
A bumper is, for example, provided at a front end of a vehicle. The bumper absorbs an impact energy when the vehicle collides with an object. A bumper configured by a reinforcement beam which is totally quenched and a pair of supporting members supporting the reinforcement beam therebetween is proposed. This kind of bumpers absorbs the impact energy by plastically deforming or crashing the reinforcement beam or the pair of supporting members. Since a force-stroke curve and a total impact energy absorption amount change in accordance with how the reinforcement beam deforms or crashes, a structure and configuration of the reinforcement beam are researched as in U.S. Pat. No. 6,361,092 B1 and US 2008/0054656 A1. The force-stroke curve shows a relationship between a displaced amount (a retracting amount) of the reinforcement beam and a load required to cause the reinforcement beam to displace at the displaced amount. The impact energy absorption amount is calculated by following equation. Impact energy=load×displaced amount.
U.S. Pat. No. 6,361,092 B1 discloses frame rails 12 and a bumper beam 7 supported between the frame rails. Cutouts 44, 48, 52 are provided between a front end and a rear end of the bumper beam. According to claim 1, webs are formed between cutouts, and the cutouts cause the rail to plastically deform in a first section connected with the frame rail in the front end of the bumper beam. It is assumed that the bumper beam is made of metal.
US 2008/0054656 A1 discloses a bumper system including a roll-formed tubular beam 21 with bracket mounts 22. The bracket mount 22 are welded to a rear surface of the roll-formed tubular beam 21. The tubular beam is formed from ultra-high-strength steel of at least 80 ksi tensile strength. The beam includes a center section, corner-forming end sections, and mounting sections intervening the center section and the end sections. At least one portion of one of the center section, the end sections, and the mounting sections is annealed to have reduced tensile strength lower than 80 ksi. According to the description in paragraph 0015, by annealing a rear wall portion of a cross section at the mounts 22 and not annealing a front wall portion, the beam is able to better maintain a shape of its front surface while reducing transmission of peaks in an impact energy to a vehicle frame.
Safety standards are established as for a collision between a bumper and an object. According to one of the safety standards, the collision is classed as a light collision often observed at a low speed and a heavy collision often observed at a high speed. The bumper is expected to absorb as large energy as it can with respect to both the light collision and the heavy collision; expected not to bend and protrude backward at a portion where an obstacle collides; and expected not to displace backward in this protruding state so much in order to prevent the bumper from damaging members configuring vehicles such as a radiator. In order to achieve this, with respect to the light collision, it is desired that a reinforcement beam do not displace backward, the reinforcement beam only deform and this deformation do not affect a pair of support members supporting the reinforcement beam therebetween. With respect to the heavy collision, it is desired that both of the reinforcement beam and the pair of support members plastically deform or crash and an impact energy do not affect a frame of a vehicle.
In the bumper of U.S. Pat. No. 6,361,092 B1, the cutouts are formed on a plane called a first surface between the front end of the bumper and the rear end of the bumper to reduce a strength of the first section connected with the frame rail and facilitate it to plastically deform. The cutouts on the first surface cause an the first surface to crack from its inner edge by an impact, or prevent the bumper from smoothly plastically deforming or crashing due to a lack of a mechanical continuity on the first surface. This results in reducing the gross energy adsorption amount and failing to prevent the reinforcement beam from displacing backward.
In the bumper of US 2008/0054656 A1, the rear wall portion of a cross section at the mounts is integrally annealed with other portions. The plastic deformation or clash cannot be precisely controlled with annealing. It is unclear that enough gross energy adsorption amount is obtained and that the reinforcement beam is successfully prevented from displacing backward. Since the heated rear wall portion is needed to be slowly chilled in annealing, a productivity of the bumper of US 2008/0054656 A1 is rather low, whereas vehicle members are expected to be manufactured in large volume.
To sum up, the bumpers disclosed in U.S. Pat. No. 6,361,092 B1 and US 2008/0054656 A1 have problems in that the gross energy adsorption amount is reduced, and in that displacement of the reinforcement beam to the backward is not well prevented. Further, US 2008/0054656 A1 has a problem in the productivity. In view of these problems, the present invention provides a bumper which is not problematic in productivity. Further, the present invention provides a bumper in which support members are not affected by a plastic deformation or crash of a reinforcement beam while undergoing the light collision; a large gross energy adsorption amount is obtained by plastically deforming or crashing both of the reinforcement beam and the supporting members while undergoing the heavy collision; and the reinforcement beam is successfully prevented from displacing backward.